1. Field of the Invention
The present invention relates to securing a rotating shaft in a sleeve bearing. More specifically, the present invention relates to a rotating device capable of being operated at multiple speeds and including means for assuring a lubricant film between the sleeve bearing and the shaft for prolonging sleeve bearing life.
2. Description of the Prior Art
In an air conditioning system it is desirable to have the capability of operating the air conditioning system to cool an enclosure notwithstanding the outdoor ambient temperature is relatively low. In a typical refrigeration circuit the condensing temperature and pressure and consequently the evaporator temperature are a function of the ambient air temperature. In order to allow air conditioning systems to operate effectively in low outdoor conditions it is necessary that the condensing temperature, the temperature at which refrigerant changes state from a gas to a liquid within the condenser, be maintained at a minimum temperature level. On hot days condenser fans are utilized to circulate ambient air in heat exchange relation with refrigerant in the condenser to effect a transfer of heat energy from the refrigerant to the air such that the refrigerant changes state from a gas to a liquid. When the ambient air temperature is reduced, a smaller volume of air or none at all needs to be forcibly circulated over the condenser surface to provide for appropriate heat transfer. If the condenser fans are operated at full speed and the ambient temperature is sufficiently low then the refrigerant is condensed at a lower temperature and thereafter subcooled to an even lower temperature such that freezeup may occur on the evaporator heat transfer surfaces. Overall system capacity may also be reduced since the condensing pressure is reduced and as a result thereof the mass flow rate of refrigerant through the system is also reduced.
To maintain the air conditioning system at full capacity with low outdoor ambient temperature conditions it is desirable to vary the speed of the condensing fan motors to vary the air flow in heat transfer relationship with the condenser. The fan speeds are varied to maintain the condensing temperature at a desired level. Typically, a refrigerant temperature is sensed someplace in the closed refrigeration circuit to determine at what flow rate to circulate air, if any, over the condensing surface.
Speed control devices have been utilized to vary the speed of the electric motor connected to drive a propeller type fan for circulating the ambient air over the condenser. It has been found that an economical and efficient system for accomplishing such a fan speed variance is to utilize a tapped transformer for supplying power at various voltages to the motor. This tapped transformer allows the use of a standard motor to achieve the various operating speeds.
Another system for utilizing a conventional economical fan motor is to utilize a motor having a sleeve bearing as opposed to a more expensive ball bearing. The sleeve bearing motor has a rotating shaft which is secured within a cylindrical sleeve. Lubricant is supplied to the space between the sleeve and the shaft for supporting the shaft on a lubricant film between the bearing and shaft.
It has been found that if the fan motor is operated at a sufficiently slow speed that the lubricant film is not established between the shaft and the bearing. If no lubricant film is established the bearing life is greatly reduced. It has further been found that if, upon startup, the shaft is operated at a minimum rotational velocity to establish a lubricant film then subsequent slower speed operation does not destroy the lubricant film. Hence, operation of the fan motor at the reduced speed after the film has been established maintains the film and acts to prolong the bearing life.
As described herein, there will be disclosed a control system which, upon startup, acts to energize the fan motor at a minimum speed which may vary from 500 to 800 revolutions per minute for a predetermined time interval to establish a lubricant film. Thereafter, depending upon the need for ambient air to be circulated over the condenser, the fan speed may be lowered while the lubricant film is maintained.